Date post: | 07-Apr-2018 |
Category: |
Documents |
Upload: | sudharsananprs |
View: | 218 times |
Download: | 0 times |
of 21
8/6/2019 Groundwater Assessment Met Bod Logy
1/21
1
GROUNDWATER ASSESSMENT METHODOLOGY
C. P. Kumar
Scientist E1
National Institute of Hydrology
Roorkee 247667 (Uttaranchal)
1.0 INTRODUCTION
Rapid industrial development, urbanisation and increase in agricultural production have
led to freshwater shortages in many parts of the world. In view of increasing demand of water
for various purposes like agricultural, domestic and industrial etc., a greater emphasis is being
laid for a planned and optimal utilisation of water resources. The water resources of the basinsremain almost constant while the demand for water continues to increase. The utilisable water
resources of India are estimated to be 112 mham out of which 69 mham is surface water
resources and 43 mham is groundwater resources.
Due to uneven distribution of rainfall both in time and space, the surface water resources
are unevenly distributed. Also, increasing intensities of irrigation from surface water alone may
result in alarming rise of water table creating problems of water-logging and salinisation,
affecting crop growth adversely and rendering large areas unproductive. This has resulted in
increased emphasis on development of groundwater resources. The simultaneous development
of groundwater, specially through dug wells and shallow tubewells, will lower water table,
provide vertical drainage and thus can prevent water-logging and salinisation. Areas, which are
already waterlogged, can also be reclaimed.
On the other hand, continuous increased withdrawals from a groundwater reservoir in
excess of replenishable recharge may result in regular lowering of water table. In such a situation,
a serious problem is created resulting in drying of shallow wells and increase in pumping head
for deeper wells and tubewells. This has led to emphasis on planned and optimal development
of water resources. An appropriate strategy will be to develop water resources with planning
based on conjunctive use of surface water and groundwater.
For a sustainable development of water resources, it is imperative to make a quantitativeestimation of the available water resources. For this, the first task would be to make a realistic
assessment of the surface water and groundwater resources and then plan their use in such a way
that full crop water requirements are met and there is neither water-logging nor excessive
lowering of groundwater table. It is necessary to maintain the groundwater reservoir in a state of
dynamic equilibrium over a period of time and the water level fluctuations have to be kept within
a particular range over the monsoon and non-monsoon seasons.
Groundwater is a dynamic system. The total annual replenishable resource is around 43
mham. Inspite of the national scenario on the availability of groundwater being favourable, there
are many areas in the country facing scarcity of water. This is because of the unplanned
8/6/2019 Groundwater Assessment Met Bod Logy
2/21
2
groundwater development resulting in fall of water levels, failure of wells, and salinity ingress
in coastal areas. The development and over-exploitation of groundwater resources in certain parts
of the country have raised the concern and need for judicious and scientific resource management
and conservation.
The National Water Policy adopted by the Government of India in 1987 and revised in2002, regards water as one of the most crucial elements in developmental planning. Regarding
groundwater, it recommends that
There should be a periodical reassessment of the groundwater potential on a scientific basis,taking into consideration the quality of the water available and economic viability of its
extraction.
Exploitation of groundwater resources should be so regulated as not to exceed the rechargingpossibilities, as also to ensure social equity. The detrimental environmental consequences of
over-exploitation of groundwater need to be effectively prevented by the Central and StateGovernments. Groundwater recharge projects should be developed and implemented for
improving both the quality and availability of groundwater resource.
Integrated and coordinated development of surface water and groundwater resources and theirconjunctive use, should be envisaged right from the project planning stage and should form
an essential part of the project implementation.
Over-exploitation of groundwater should be avoided especially near the coast to preventingress of seawater into sweet water aquifers.
A complexity of factors - hydrogeological, hydrological and climatological, control the
groundwater occurrence and movement. The precise assessment of recharge and discharge is
rather difficult, as no techniques are currently available for their direct measurements. Hence, the
methods employed for groundwater resource estimation are all indirect. Groundwater being a
dynamic and replenishable resource, is generally estimated based on the component of annual
recharge, which could be subjected to development by means of suitable groundwater structures.
For quantification of groundwater resources, proper understanding of the behaviour and
characteristics of the water bearing rock formation, known as aquifer, is essential. An aquifer has
two main functions - (i) to transit water (conduit function) and (ii) to store it (storage function).
The groundwater resources in unconfined aquifers can be classified as static and dynamic. Thestatic resources can be defined as the amount of groundwater available in the permeable portion
of the aquifer below the zone of water level fluctuation. The dynamic resources can be defined
as the amount of groundwater available in the zone of water level fluctuation. The replenishable
groundwater resource is essentially a dynamic resource which is replenished annually or
periodically by precipitation, irrigation return flow, canal seepage, tank seepage, influent seepage,
etc.
The methodologies adopted for computing groundwater resources, are generally based
on the hydrologic budget techniques. The hydrologic equation for groundwater regime is a
specialized form of water balance equation that requires quantification of the components of
8/6/2019 Groundwater Assessment Met Bod Logy
3/21
3
inflow to and outflow from a groundwater reservoir, as well as changes in storage therein. Some
of these are directly measurable, few may be determined by differences between measured
volumes or rates of flow of surface water, and some require indirect methods of estimation.
Water balance techniques have been extensively used to make quantitative estimates of
water resources and the impact of mans activities on the hydrological cycle. The study of waterbalance requires the systematic presentation of data on the water supply and its use within a given
study area for a specific period. The water balance of an area is defined by the hydrologic
equation, which is basically a statement of the law of conservation of mass as applied to the
hydrological cycle. With water balance approach, it is possible to evaluate quantitatively
individual contribution of sources of water in the system, over different time periods, and to
establish the degree of variation in water regime due to changes in components of the system.
A basinwise approach yields the best results where the groundwater basin can be
characterized by prominent drainages. A thorough study of the topography, geology and aquifer
conditions should be taken up. The limit of the groundwater basin is controlled not only bytopography but also by the disposition, structure and permeability of rocks and the configuration
of the water table.
Generally, in igneous and metamorphic rocks, the surface water and groundwater basins
are coincident for all practical purposes, but marked differences may be encountered in stratified
sedimentary formations. Therefore, the study area for groundwater balance study is preferably
taken as a doab which is bounded on two sides by two streams and on the other two sides by
other aquifers or extension of the same aquifer. Once the study area is identified, comprehensive
studies can be undertaken to estimate for selected period of time, the input and output of water,
and change in storage to draw up water balance of the basin.
The estimation of groundwater balance of a region requires quantification of all
individual inflows to or outflows from a groundwater system and change in groundwater storage
over a given time period. The basic concept of water balance is:
Input to the system - outflow from the system = change in storage of the system
(over a period of time)
The general methodology of computing groundwater balance consists of the following:
Identification of significant components, Evaluating and quantifying individual components, and Presentation in the form of water balance equation.The groundwater balance study of an area may serve the following purposes:
As a check on whether all flow components involved in the system have beenquantitatively accounted for, and what components have the greatest bearing on the
problem under study.
To calculate one unknown component of the groundwater balance equation, providedall other components are quantitatively known with sufficient accuracy.
8/6/2019 Groundwater Assessment Met Bod Logy
4/21
4
As a model of the hydrological processes under study, which can be used to predictthe effect that changes imposed on certain components will have on the other
components of groundwater system.
2.0 GROUNDWATER BALANCE EQUATION
Considering the various inflow and outflow components in a given study area, the
groundwater balance equation can be written as:
Rr + Rc + Ri + Rt + Si + Ig = Et + Tp + Se + Og + S (1)
where,
Rr = recharge from rainfall;
Rc = recharge from canal seepage;
Ri = recharge from field irrigation;Rt = recharge from tanks;
Si = influent seepage from rivers;
Ig = inflow from other basins;
Et = evapotranspiration from groundwater;
Tp = draft from groundwater;
Se = effluent seepage to rivers;
Og = outflow to other basins; and
S = change in groundwater storage.
Preferably, all elements of the groundwater balance equation should be computed using
independent methods. However, it is not always possible to compute all individual components
of the groundwater balance equation separately. Sometimes, depending on the problem, some
components can be lumped, and account only for their net value in the equation.
Computations of various components usually involve errors, due to shortcomings in the
estimation techniques. The groundwater balance equation therefore generally does not balance,
even if all its components are computed by independent methods. The resultant discrepancy in
groundwater balance is defined as a residual term in the balance equation, which includes errors
in the quantitative determination of various components as well as values of the components
which have not been accounted in the equation.
The water balance may be computed for any time interval. The complexity of the
computation of the water balance tends to increase with increase in area. This is due to a related
increase in the technical difficulty of accurately computing the numerous important water balance
components.
3.0 DATA REQUIREMENTS FOR A GROUNDWATER BALANCE STUDY
For carrying out a groundwater balance study, following data may be required over a
given time period:
8/6/2019 Groundwater Assessment Met Bod Logy
5/21
5
Rainfall data: Monthly rainfall data of sufficient number of rainguage stations lying within or
around the study area, along with their locations, should be available.
Land use data and cropping patterns: Land use data are required for estimating the
evapotranspiration losses from the water table through forested area. Cropping pattern data are
necessary for estimating the spatial and temporal distributions of groundwater withdrawals, ifrequired. Monthly pan evaporation rates should also be available at few locations for estimation
of consumptive use requirements of different crops.
River data: Monthly river stage and discharge data along with river cross-sections are required
at few locations for estimating the river-aquifer interflows.
Canal data: Monthwise water releases into the canal and its distributories along with running
days during each month are required. To account for the seepage losses through the canal system,
the seepage loss test data are required in different canal reaches and distributories.
Tank data: Monthly tank gauges and water releases should be available. In addition, depth vs
area and depth vs capacity curves should also be available for computing the evaporation and
seepage losses from tanks. Field test data are required for computing infiltration capacity to be
used to evaluate the recharge from depression storage.
Water table data: Monthly water table data (or at least pre-monsoon and post-monsoon data)
from sufficient number of well-distributed observation wells along with their locations are
required. The available data should comprise reduced level (R.L.) of water table and depth to
water table.
Groundwater draft: For estimating groundwater withdrawals, the number of each type of wellsoperating in the area, their corresponding running hours each month and discharge are required.
If a complete inventory of wells is not available, then this can be obtained by carrying out sample
surveys.
Aquifer parameters: Data regarding the storage coefficient and transmissivity are required at
sufficient number of locations in the study area.
4.0 GROUNDWATER RESOURCE ESTIMATION METHODOLOGY
The Groundwater Estimation Committee (GEC) was constituted by the Government ofIndia in 1982 to recommend methodologies for estimation of the groundwater resource potential
in India. It was recommended by the committee that the groundwater recharge should be
estimated based on groundwater level fluctuation method. However, in areas, where groundwater
level monitoring is not being done regularly, or where adequate data about groundwater level
fluctuation is not available, adhoc norms of rainfall infiltration may be adopted. In order to
review the recommended methodology, the Committee was reconstituted in 1995, which released
its report in 1997. This Committee proposed several improvements in the existing methodology
based on groundwater level fluctuation approach. Salient features of their recommendations are
given below.
8/6/2019 Groundwater Assessment Met Bod Logy
6/21
6
(a) Watershed may be used as the unit for groundwater resource assessment in hard rockareas, which occupies around 2/3rd part of the country. The size of the watershed as a
hydrological unit could be of about 100 to 300 sq. km. area. The assessment made for
watershed as unit may be transferred to administrative unit such as block, for planning
development programmes.
(b) For alluvial areas, the present practice of assessment based on block/taluka/mandal-wisebasis is retained. The possibility of adopting doab as the unit of assessment in alluvial
areas needs further detailed studies.
(c) The total geographical area of the unit for resource assessment is to be divided into
subareas such as hilly regions (slope > 20%), saline groundwater areas, canal command
areas and non-command areas, and separate resource assessment may be made for these
subareas. Variations in geomorphological and hydrogeological characteristics may be
considered within the unit.
(d) For hard rock areas, the specific yield value may be estimated by applying the water levelfluctuation method for the dry season data, and then using this specific yield value in the
water level fluctuation method for the monsoon season to get recharge. For alluvial areas,
specific yield values may be estimated from analysis of pumping tests. However, norms
for specific yield values in different hydrogeological regions may still be necessary for
use in situations where the above methods are not feasible due to inadequacy of data.
(e) There should be at least 3 spatially well-distributed observation wells in the unit, or one
observation well per 100 sq. km. whichever is more.
(f) The problem of accounting for groundwater inflow/outflow and base flow from a regionis difficult to solve. If watershed is used as a unit for resource assessment in hard rock
areas, the groundwater inflow/outflow may become negligible. The base flow can be
estimated if one stream gauging station is located at the exit of the watershed.
(g) Norms for return flow from groundwater and surface water irrigation are revised takinginto account the source of water (groundwater/surface water), type of crop
(paddy/non-paddy) and depth of groundwater level.
In subsequent section, the recommended GEC norms for estimation of various
inflow/outflow components of the groundwater balance equation have been mentioned atappropriate places, along with other methodologies/formulae in use.
5.0 ESTIMATION OF GROUNDWATER BALANCE COMPONENTS
The various inflow/outflow components of the groundwater balance equation may be
estimated through appropriate empirical relationships suitable for a region, Groundwater
Estimation Committee norms (1997), field experiments or other methods, as discussed below.
8/6/2019 Groundwater Assessment Met Bod Logy
7/21
7
5.1 Recharge from Rainfall (Rr)
Rainfall is the major source of recharge to groundwater. Part of the rain water, that falls
on the ground, is infiltrated into the soil. A part of this infiltrated water is utilized in filling the
soil moisture deficiency while the remaining portion percolates down to reach the water table,
which is termed as rainfall recharge to the aquifer. The amount of rainfall recharge depends onvarious hydrometeorological and topographic factors, soil characteristics and depth to water
table. The methods for estimation of rainfall recharge involve the empirical relationships
established between recharge and rainfall developed for different regions, Groundwater Resource
Estimation Committee norms, groundwater balance approach, and soil moisture data based
methods.
5.1.1 Empirical Methods
Several empirical formulae have been worked out for various regions in India on the basis
of detailed studies. Some of the commonly used formulae are:
(a) Chaturvedi formula: Based on the water level fluctuations and rainfall amounts in
Ganga-Yamuna doab, Chaturvedi in 1936, derived an empirical relationship to arrive at the
recharge as a function of annual precipitation.
Rr = 2.0 (P - 15)0.4
(2)
where,
Rr = net recharge due to precipitation during the year, in inches; and
P = annual precipitation, in inches.
This formula was later modified by further work at the U.P. Irrigation Research Institute,
Roorkee and the modified form of the formula is
Rr = 1.35 (P - 14)0.5
(3)
The Chaturvedi formula has been widely used for preliminary estimations of groundwater
recharge due to rainfall. It may be noted that there is a lower limit of the rainfall below which the
recharge due to rainfall is zero. The percentage of rainfall recharged commences from zero at P
= 14 inches, increases upto 18% at P = 28 inches, and again decreases. The lower limit of rainfall
in the formula may account for the soil moisture deficit, the interception losses and potentialevaporation. These factors being site specific, one generalized formula may not be applicable to
all the alluvial areas. Tritium tracer studies on groundwater recharge in the alluvial deposits of
Indo-Gangetic plains of western U.P., Punjab, Haryana and alluvium in Gujarat state have
indicated variations with respect to Chaturvedi formula.
(b) Kumar and Seethapathi (2002): They conducted a detailed seasonal groundwater balance
study in Upper Ganga Canal command area for the period 1972-73 to 1983-84 to determine
groundwater recharge from rainfall. It was observed that as the rainfall increases, the quantity of
recharge also increases but the increase is not linearly proportional. The recharge coefficient
(based upon the rainfall in monsoon season) was found to vary between 0.05 to 0.19 for the study
8/6/2019 Groundwater Assessment Met Bod Logy
8/21
8
area. The following empirical relationship (similar to Chaturvedi formula) was derived by fitting
the estimated values of rainfall recharge and the corresponding values of rainfall in the monsoon
season through the non-linear regression technique.
Rr = 0.63 (P - 15.28)0.76 (4)
where,
Rr = Groundwater recharge from rainfall in monsoon season (inch);
P = Mean rainfall in monsoon season (inch).
The relative errors (%) in the estimation of rainfall recharge computed from the
proposed empirical relationship was compared with groundwater balance study. In almost all the
years, the relative error was found to be less than 8%. On the other hand, relative errors (%)
computed from Chaturvedi formula (equations 2 and 3) were found to be quite high. Therefore,
equation (4) can conveniently be used for better and quick assessment of natural groundwaterrecharge in Upper Ganga Canal command area.
(c) Amritsar formula: Using regression analysis for certain doabs in Punjab, the Irrigation and
Power Research Institute, Amritsar, developed the following formula in 1973.
Rr = 2.5 (P - 16)0.5
(5)
where, Rr and P are measured in inches.
(d) Krishna Rao: Krishna Rao gave the following empirical relationship in 1970 to determine
the groundwater recharge in limited climatological homogeneous areas:
Rr = K (P - X) (6)
The following relation is stated to hold good for different parts of Karnataka:
Rr = 0.20 (P - 400) for areas with annual normal rainfall (P) between 400 and 600 mm
Rr = 0.25 (P - 400) for areas with P between 600 and 1000 mm
Rr = 0.35 (P - 600) for areas with P above 2000 mm
where, Rr and P are expressed in millimetres.
The relationships indicated above, which were tentatively proposed for specific
hydrogeological conditions, have to be examined and established or suitably altered for
application to other areas.
8/6/2019 Groundwater Assessment Met Bod Logy
9/21
9
5.1.2 Groundwater Resource Estimation Committee Norms
If adequate data of groundwater levels are not available, rainfall recharge may be
estimated using the rainfall infiltration method. The same recharge factor may be used for both
monsoon and non-monsoon rainfall, with the condition that the recharge due to non-monsoon
rainfall may be taken as zero, if the rainfall during non-monsoon season is less than 10% ofannual rainfall. Groundwater Resource Estimation Committee (1997) recommended the
following rainfall infiltration factors:
(a) Alluvial areas
Indo-Gangetic and inland areas - 22 %
East coast - 16 %
West coast - 10 %
(b) Hard rock areas
Weathered granite, gneiss and
schist with low clay content - 11 %
Weathered granite, gneiss and
schist with significant clay content - 8 %
Granulite facies like charnockite etc. - 5 %
Vesicular and jointed basalt - 13 %
Weathered basalt - 7 %
Laterite - 7 %Semiconsolidated sandstone - 12 %
Consolidated sandstone, Quartzites,
Limestone (except cavernous limestone) - 6 %
Phyllites, Shales - 4 %
Massive poorly fractured rock - 1 %
An additional 2% of rainfall recharge factor may be used in areas where watershed
development with associated soil conservation measures are implemented. This additional factoris separate from contribution due to water conservation structures such as check dams, nalla
bunds, percolation tanks etc., for which the norms are defined separately.
5.1.3 Groundwater Balance Approach
In this method, all components of the groundwater balance equation (1), except the
rainfall recharge, are estimated individually. The algebraic sum of all input and output
components is equated to the change in groundwater storage, as reflected by the water table
fluctuation, which in turn yields the single unknown in the equation, namely, the rainfall
recharge. A pre-requisite for successful application of this technique is the availability of very
8/6/2019 Groundwater Assessment Met Bod Logy
10/21
10
extensive and accurate hydrological and meteorological data. The groundwater balance approach
is valid for the areas where the year can be divided into monsoon and non-monsoon seasons with
the bulk of rainfall occurring in former.
Groundwater balance study for monsoon and non-monsoon periods is carried out
separately. The former yields an estimate of recharge coefficient and the later determines thedegree of accuracy with which the components of water balance equation have been estimated.
Alternatively, the average specific yield in the zone of fluctuation can be determined from a
groundwater balance study for the non-monsoon period and using this specific yield, the recharge
due to rainfall can be determined using the groundwater balance components for the monsoon
period.
5.1.4 Soil Moisture Data Based Methods
Soil moisture data based methods are the lumped and distributed model and the nuclear
methods. In the lumped model, the variation of soil moisture content in the vertical direction isignored and any effective input into the soil is assumed to increase the soil moisture content
uniformly. Recharge is calculated as the remainder when losses, identified in the form of runoff
and evapotranspiration, have been deducted from the precipitation with proper accounting of soil
moisture deficit. In the distributed model, variation of soil moisture content in the vertical
direction is accounted and the method involves the numerical solution of partial differential
equation (Richards equation) governing one-dimensional flow through unsaturated medium, with
appropriate initial and boundary conditions.
(a) Soil Water Balance Method
Water balance models were developed in the 1940s by Thornthwaite (1948) and revisedby Thornthwaite and Mather (1955). The method is essentially a book-keeping procedure which
estimates the balance between the inflow and outflow of water. When applying this method to
estimate the recharge for a catchment area, the calculation should be repeated for areas with
different precipitation, evapotranspiration, crop type and soil type. The soil water balance method
is of limited practical value, because evapotranspiration is not directly measurable. Moreover,
storage of moisture in the unsaturated zone and the rates of infiltration along the various possible
routes to the aquifer form important and uncertain factors. Another aspect that deserves attention
is the depth of the root zone which may vary in semi-arid regions between 1 and 30 meters.
Results from this model are of very limited value without calibration and validation, because of
the substantial uncertainty in input data.
(b) Nuclear Methods
Nuclear techniques can be used for the determination of recharge by measuring the travel
of moisture through a soil column. The technique is based upon the existence of a linear relation
between neutron count rate and moisture content (% by volume) for the range of moisture
contents generally occurring in the unsaturated soil zone. The mixture of Beryllium (Be) and
Radium (Ra) is taken as the source of neutrons. Another method is the gamma ray transmission
method based upon the attenuation of gamma rays in a medium through which it passes. The
extent of attenuation is closely linked with moisture content of the soil medium.
8/6/2019 Groundwater Assessment Met Bod Logy
11/21
11
5.2 Recharge from Canal Seepage (Rc)
Seepage refers to the process of water movement from a canal into and through the bed
and wall material. Seepage losses from irrigation canals often constitute a significant part of the
total recharge to groundwater system. Hence, it is important to properly estimate these losses forrecharge assessment to groundwater system. Recharge by seepage from canals depend upon the
size and cross-section of the canal, depth of flow, characteristics of soils in the bed and sides, and
location as well as level of drains on either side of the canal. A number of empirical formulae and
formulae based on theoretical considerations have been proposed to estimate the seepage losses
from canals.
Recharge from canals that are in direct hydraulic connection with a phreatic aquifer
underlain by a horizontal impermeable layer at shallow depth, can be determined by Darcy's
equation, provided the flow satisfies Dupuit assumptions.
...(7)1 AL
hhKR sc
=
where, hs and hl are water-level elevations above the impermeable base, respectively, at the canal,
and at distance L from it. For calculating the area of flow cross-section, the average of the
saturated thickness (hs + hl)/2 is taken. The crux of computation of seepage depends on correct
assessment of the hydraulic conductivity, K. Knowing the percentage of sand, silt and clay, the
hydraulic conductivity of undisturbed soil can be approximately determined using the soil
classification triangle showing relation of hydraulic conductivity to texture for undisturbed
sample (Johnson, 1963).
A number of investigations have been carried out to study the seepage losses from canals.
The following formulae/values are in vogue for the estimation of seepage losses:
(a) As reported by the Indian Standard (IS: 9452 Part 1, 1980), the loss of water by seepage
from unlined canals in India varies from 0.3 to 7.0 cumec per million square meter of
wetted area depending on the permeability of soil through which the canal passes,
location of water table, distance of drainage, bed width, side slope, and depth of water
in the canal. Transmission loss of 0.60 cumec per million square meter of wetted area of
lined canal is generally assumed (IS: 10430, 1982).
(b) For unlined channels in Uttar Pradesh, it has been proposed that the losses per million
square meter of wetted area are 2.5 cumec for ordinary clay loam to about 5 cumec for
sandy loam with an average of 3 cumec. Empirically, the seepage losses can be computed
using the following formula:
where, B and D are the bed width and depth, respectively, of the channel in meters, C is
a constant with a value of 1.0 for intermittent running channels and 0.75 for continuous
)8...()(200
/3/2
DBC
kmcumecsinLosses +=
8/6/2019 Groundwater Assessment Met Bod Logy
12/21
12
running channels.
(c) For lined channels in Punjab, the following formula is used for estimation of seepage
losses:
Rc = 1.25 Q0.56 (9)
where, Rc is the seepage loss in cusec per million square foot of wetted perimeter and Q,
in cusec, is the discharge carried by the channel. In unlined channels, the loss rate on an
average is four times the value computed using the above formula.
(d) U. S. B. R. recommended the channel losses based on the channel bed material as given
below:
Material Seepage Losses
(cumec per million square meter of wetted area)
Clay and clay loam : 1.50
Sandy loam : 2.40
Sandy and gravely soil : 8.03
Concrete lining : 1.20
(e) Groundwater Resource Estimation Committee (1997) has recommended the following
norms:
(i) Unlined canals in normal soil with some clay content along with sand- 1.8 to 2.5 cumec per million square meter of wetted area.
(ii) Unlined canals in sandy soil with some silt content- 3.0 to 3.5 cumec per million square meter of wetted area.
(iii) Lined canals and canals in hard rock areas- 20% of the above values for unlined canals.
These values are valid if the water table is relatively deep. In shallow water table and
water logged areas, the recharge from canal seepage may be suitably reduced. Specific results
from case studies may be used, if available. The above norms take into consideration the type ofsoil in which the canal runs while computing seepage. However, the actual seepage will also be
controlled by the width of canal (B), depth of flow (D), hydraulic conductivity of the bed material
(K) and depth to water table.
Knowing the values of B and D, the range of seepage losses (Rc_max and Rc_min) from the
canal may be obtained as
Rc_max = K (B + 2D) (in case of deeper water table) (10a)
Rc_min = K (B - 2D) (in case of water table at the level of channel bed) (10b)
8/6/2019 Groundwater Assessment Met Bod Logy
13/21
13
However, the various guidelines for estimating losses in the canal system, as given above,
are at best approximate. Thus, the seepage losses may best be estimated by conducting actual
tests in the field. The methods most commonly adopted are:
Inflow - outflow method: In this method, the water that flows into and out of the section ofcanal, under study, is measured using current meter or Parshall flume method. The difference
between the quantities of water flowing into and out of the canal reach is attributed to seepage.
This method is advantageous when seepage losses are to be measured in long canal reaches with
few diversions.
Ponding method: In this method, bunds are constructed in the canal at two locations, one
upstream and the other downstream of the reach of canal with water filled in it. The total change
in storage in the reach is measured over a period of time by measuring the rate of drop of water
surface elevation in the canal reach. Alternatively, water may be added to maintain a constant
water surface elevation. In this case, the volume of water added is measured along with theelapsed time to compute the rate of seepage loss. The ponding method provides an accurate
means of measuring seepage losses and is especially suitable when they are small (e.g. in lined
canals).
Seepage meter method: The seepage meter is a modified version of permeameter developed for
use under water. Various types of seepage meters have been developed. The two most important
are seepage meter with submerged flexible water bag and falling head seepage meter. Seepage
meters are suitable for measuring local seepage rates in canals or ponds and used only in unlined
or earth-lined canals. They are quickly and easily installed and give reasonably satisfactory
results for the conditions at the test site but it is difficult to obtain accurate results when seepage
losses are low.
The total losses from the canal system generally consist of the evaporation losses (Ec) and
the seepage losses (Rc). The evaporation losses are generally 10 to 15 percent of the total losses.
Thus the Rc value is 85 to 90 percent of the losses from the canal system.
5.3 Recharge from Field Irrigation (Ri)
Water requirements of crops are met, in parts, by rainfall, contribution of moisture from
the soil profile, and applied irrigation water. A part of the water applied to irrigated field crops
is lost in consumptive use and the balance infiltrates to recharge the groundwater. The processof re-entry of a part of the water used for irrigation is called return flow. Percolation from applied
irrigation water, derived both from surface water and groundwater sources, constitutes one of the
major components of groundwater recharge. The irrigation return flow depends on the soil type,
irrigation practice and type of crop. Therefore, irrigation return flows are site specific and will
vary from one region to another.
For a correct assessment of the quantum of recharge by applied irrigation, studies are
required to be carried out on experimental plots under different crops in different seasonal
conditions. The method of estimation comprises application of the water balance equation
involving input and output of water in experimental fields.
8/6/2019 Groundwater Assessment Met Bod Logy
14/21
14
The recharge due to irrigation return flow may also be estimated, based on the source of
irrigation (groundwater or surface water), the type of crop (paddy, non-paddy) and the depth of
water table below ground surface, using the norms provided by Groundwater Resource
Estimation Committee (1997), as given below (as percentage of water application):
Source of Type of Water table below ground surface
Irrigation Crop 25m
Groundwater Non-paddy 25 15 5
Surface water Non-paddy 30 20 10
Groundwater Paddy 45 35 20
Surface water Paddy 50 40 25
For surface water, the recharge is to be estimated based on water released at the outlet
from the canal/distribution system. For groundwater, the recharge is to be estimated based ongross draft. Where continuous supply is used instead of rotational supply, an additional recharge
of 5% of application may be used. Specific results from case studies may be used, if available.
5.4 Recharge from Tanks (Rt)
Studies have indicated that seepage from tanks varies from 9 to 20 percent of their live
storage capacity. However, as data on live storage capacity of large number of tanks may not be
available, seepage from the tanks may be taken as 44 to 60 cm per year over the total water
spread, taking into account the agro-climatic conditions in the area. The seepage from percolation
tanks is higher and may be taken as 50 percent of its gross storage. In case of seepage from ponds
and lakes, the norms as applied to tanks may be taken. Groundwater Resource EstimationCommittee (1997) has recommended that based on the average area of water spread, the recharge
from storage tanks and ponds may be taken as 1.4 mm/day for the period in which tank has water.
If data on the average area of water spread is not available, 60% of the maximum water spread
area may be used instead of average area of water spread.
In case of percolation tanks, recharge may be taken as 50% of gross storage, considering
the number of fillings, with half of this recharge occurring in monsoon season and the balance
in non-monsoon season. Recharge due to check dams and nala bunds may be taken as 50% of
gross storage (assuming annual desilting maintenance exists) with half of this recharge occurring
in the monsoon season and the balance in the non-monsoon season.
5.5 Influent and Effluent Seepage (Si & Se)
The river-aquifer interaction depends on the transmissivity of the aquifer system and the
gradient of water table in respect to the river stage. Depending on the water level in the river and
in the aquifer (in the vicinity of river), the river may recharge the aquifer (influent) or the aquifer
may contribute to the river flow (effluent). The effluent or influent character of the river may vary
from season to season and from reach to reach. The seepage from/to the river can be determined
by dividing the river reach into small sub-reaches and observing the discharges at the two ends
of the sub-reach along with the discharges of its tributaries and diversions, if any. The discharge
8/6/2019 Groundwater Assessment Met Bod Logy
15/21
15
at the downstream end is expressed as:
Qd. t = Qu. t + Qg. t + Qt. t - Qo. t - E. t Srb (11)
where,
Qd = discharge at the downstream section;
Qu = discharge at the upstream section;
Qg = groundwater contribution (unknown quantity; -ve computed value indicates influent
conditions);
Qt = discharge of tributaries;
Qo = discharge diverted from the river;
E = rate of evaporation from river water surface and flood plain (for extensive bodies of
surface water and for long time periods, evaporation from open water surfaces can not
be neglected);
Srb = change in bank storage ( + for decrease and - for increase); and t = time period.
The change in bank storage can be determined by monitoring the water table along the
cross-section normal to the river. Thus, using the above equation, seepage from/to the river over
a certain period of time t can be computed. However, this would be the contribution from
aquifers on both sides of the stream. The contribution from each side can be separated by the
following method:
(12)...Q. gRRLL
LL
TITI
TIbankleftfromonContributi
+=
(13)...Q. gRRLL
RR
TITI
TIbankrightfromonContributi
+=
where, IL and TL are gradient and transmissivity respectively on the left side and IR and TR are
those on the right.
5.6 Inflow from and Outflow to Other Basins (Ig & Og)
For the estimation of groundwater inflow/outflow from/to other basins, regional watertable contour maps are drawn based on the observed water level data from wells located within
and outside the study area. The flows into and out of a region are governed mainly by the
hydraulic gradient and transmissivity of the aquifer. The gradient can be determined by taking
the slope of the water table normal to water table contour. The length of the section, across which
groundwater inflow/outflow occurs, is determined from contour maps, the length being measured
parallel to the contour. The inflow/outflow is determined as follows:
(14)...LITOorIL
gg =
8/6/2019 Groundwater Assessment Met Bod Logy
16/21
16
where, T is the transmissivity and I is the hydraulic gradient averaged over a length L of
contour line.
5.7 Evapotranspiration from Groundwater (Et)
Evapotranspiration is the combined process of transpiration from vegetation and
evaporation from both soil and free water surfaces. Potential evapotranspiration is the maximum
loss of water through evapotranspiration. Evapotranspiration from groundwater occurs in
waterlogged areas or in forested areas with roots extending to the water table. From the land use
data, area under forests is available while the waterlogged areas may be demarcated from depth
to water table maps. The potential evapotranspiration from such areas can be computed using
standard methods.
Depth to water table maps may be prepared based on well inventory data to bring into
focus the extensiveness of shallow water table areas. During well inventory, investigation shouldbe specifically oriented towards accurately delineating water table depth for depths less than 2
meter. The evapotranspiration can be estimated based on the following equations:
Et = PEt * A if h > hs (15a)
Et = 0 if h < (hs - d) (15b)
Et = PEt * A (h - (hs - d))/d if (hs-d) h hs (15c)
where,
Et = evapotranspiration in volume of water per unit time[L3 T-1];
PEt = maximum rate of evapotranspiration in volume of water per unit area per unit time
[L3
L-2
T-1
];
A = surface area [L2];
h = water table elevation [L];
hs = water table elevation at which the evapotranspiration loss reaches the maximum
value; and
d = extinction depth. When the distance between hs and h exceeds d, evapotranspiration
from groundwater ceases [L].
5.8 Draft from Groundwater (Tp)
Draft is the amount of water lifted from the aquifer by means of various lifting devices.
To estimate groundwater draft, an inventory of wells and a sample survey of groundwater draft
from various types of wells (state tubewells, private tubewells and open wells) are required. For
state tubewells, information about their number, running hours per day, discharge, and number
of days of operation in a season is generally available in the concerned departments. To compute
the draft from private tubewells, pumping sets and rahats etc., sample surveys have to be
conducted regarding their number, discharge and withdrawals over the season.
8/6/2019 Groundwater Assessment Met Bod Logy
17/21
17
In areas where wells are energised, the draft may be computed using power consumption
data. By conducting tests on wells, the average draft per unit of electricity consumed can be
determined for different ranges in depth to water levels. By noting the depth to water level at
each distribution point and multiplying the average draft value with the number of units of
electricity consumed, the draft at each point can be computed for every month.
5.9 Change in Groundwater Storage (S)
To estimate the change in groundwater storage, the water levels are observed through a
network of observation wells spread over the area. The water levels are highest immediately after
monsoon in the month of October or November and lowest just before rainfall in the month of
May or June. During the monsoon season, the recharge is more than the extraction; therefore, the
change in groundwater storage between the beginning and end of monsoon season indicates the
total volume of water added to the groundwater reservoir. While the change in groundwater
storage between the beginning and end of the non-monsoon season indicates the total quantity
of water withdrawn from groundwater storage. The change in storage (S) is computed asfollows:
S = h A Sy (16)
where, h = change in water table elevation during the given time period;
A = area influenced by the well; and
Sy = specific yield.
Groundwater Resource Estimation Committee (1997) recommended that the size of the
watershed as a hydrological unit could be of about 100 to 300 sq. km. area and there should be
at least three spatially well-distributed observation wells in the unit, or one observation well per
100 sq. km., whichever is more. However, as per IILRI (1974), the following specification may
serve as a rough guide:
Size of the Number of Observation Number of Observation
Area (ha) Points Points per 100 hectares
100 20 20
1,000 40 4
10,000 100 1
1,00,000 300 0.3
The specific yield may be computed from pumping tests. Groundwater Resource
Estimation Committee (1997) recommended the following values of specific yield for different
geological formations:
(a) Alluvial areas
Sandy alluvium - 16.0 %
Silty alluvium - 10.0 %
Clayey alluvium - 6.0 %
8/6/2019 Groundwater Assessment Met Bod Logy
18/21
18
(b) Hard rock areas
Weathered granite, gneiss and
schist with low clay content - 3.0 %
Weathered granite, gneiss and
schist with significant clay content - 1.5 %
Weathered or vesicular, jointed basalt - 2.0 %
Laterite - 2.5 %
Sandstone - 3.0 %
Quartzites - 1.5 %
Limestone - 2.0 %
Karstified limestone - 8.0 %
Phyllites, Shales - 1.5 %Massive poorly fractured rock - 0.3 %
The values of specific yield in the zone of fluctuation of water table in different parts of
the basin can also be approximately determined from the soil classification triangle showing
relation between particle size and specific yield (Johnson, 1967).
6.0 ESTABLISHMENT OF RECHARGE COEFFICIENT
Groundwater balance study is a convenient way of establishing the rainfall recharge
coefficient, as well as to cross check the accuracy of the various prevalent methods for the
estimation of groundwater losses and recharge from other sources. The steps to be followed are:
1. Divide the year into monsoon and non-monsoon periods.
2. Estimate all the components of the water balance equation other than rainfall recharge for
monsoon period using the available hydrological and meteorological information and
employing the prevalent methods for estimation.
3. Substitute these estimates in the water balance equation and thus calculate the rainfall
recharge and hence recharge coefficient (recharge/rainfall ratio). Compare this estimate
with those given by various empirical relations valid for the area of study.
4. For non-monsoon season, estimate all the components of water balance equation
including the rainfall recharge which is calculated using recharge coefficient value
obtained through the water balance of monsoon period. The rainfall recharge (Rr) will be
of very small order in this case. A close balance between the left and right sides of the
equation will indicate that the net recharge from all the sources of recharge and discharge
has been quantified with a good degree of accuracy.
By quantifying all the inflow/outflow components of a groundwater system, one can
determine which particular component has the most significant effect on the groundwater flow
8/6/2019 Groundwater Assessment Met Bod Logy
19/21
19
regime. Alternatively, a groundwater balance study may be used to compute one unknown
component (e.g. the rainfall recharge) of the groundwater balance equation, when all other
components are known. The balance study may also serve as a model of the area under study,
whereby the effect of change in one component can be used to predict the effect of changes in
other components of the groundwater system. In this manner, the study of groundwater balance
has a significant role in planning a rational groundwater development of a region.
7.0 CONCLUDING REMARKS
Water balance approach, essentially a lumped model study, is a viable method ofestablishing the rainfall recharge coefficient and for evaluating the methods adopted
for the quantification of discharge and recharge from other sources. For proper
assessment of potential, present use and additional exploitability of water resources
at optimal level, a water balance study is necessary. It has been reported that the
groundwater resource estimation methodology recommended by Groundwater
Resource Estimation Committee (1997) is being used by most of the organisationsin India.
Groundwater exploitation should be such that protection from depletion is provided,protection from pollution is provided, negative ecological effects are reduced to a
minimum and economic efficiency of exploitation is attained. Determination of
exploitable resources should be based upon hydrological investigations. These
investigations logically necessitate use of a mathematical model of groundwater
system for analysing and solving the problems. The study of water balance is a
prerequisite for groundwater modelling.
There is a need for studying unsaturated and saturated flow through weathered andfractured rocks for finding the recharge components from rainfall and from
percolation tanks in hard rock groundwater basins. The irrigation return flow under
different soils, crops and irrigation practices need to be quantified. Assessment of
groundwater quality in many groundwater basins is a task yet to be performed. A
hydrological database for groundwater assessment should be established. Also user
friendly software should be developed for quick assessment of regional groundwater
resources. Tata Infotech Limited (India) has developed a proprietary software
GEMS (Groundwater Estimation & Management System) based upon the
recommendations of Groundwater Resource Estimation Committee (1997).
Non-conventional methods for utilisation of water such as through inter-basintransfers, artificial recharge of groundwater and desalination of brackish or sea water
as well as traditional water conservation practices like rainwater harvesting, including
roof-top rainwater harvesting, need to be practiced to further increase the utilisable
water resources.
8/6/2019 Groundwater Assessment Met Bod Logy
20/21
20
REFERENCES
1. Chandra, Satish and R. S. Saksena, 1975. "Water Balance Study for Estimation of
Groundwater Resources", Journal of Irrigation and Power, India, October 1975, pp.
443-449.
2. "Groundwater Resource Estimation Methodology - 1997". Report of the Groundwater
Resource Estimation Committee, Ministry of Water Resources, Government of India,
New Delhi, June 1997.
3. IILRI, 1974. " Drainage Principles and Applications", Survey and Investigation,
Publication 16, Vol. III.
4. Johnson, A. I., 1963. "Application of Laboratory Permeability Data", Open File Report,
U.S.G.S., Water Resources Division, Denver, Colorado, 34 p.
5. Johnson, A. I., 1967. "Specific Yield - Compilation of Specific Yields for Various
Materials", Water Supply Paper, U.S.G.S., 1962-D, 74 p.
6. Karanth, K. R., 1987. "Groundwater Assessment, Development and Management", Tata
McGraw-Hill Publishing Company Limited, New Delhi, pp. 576-657.
7. Kumar, C. P. and P. V. Seethapathi, 1988. "Effect of Additional Surface Irrigation
Supply on Groundwater Regime in Upper Ganga Canal Command Area, Part I -
Groundwater Balance", National Institute of Hydrology, Case Study Report No. CS-10
(Secret/Restricted), 1987-88.
8. Kumar, C. P. and P. V. Seethapathi, 2002. " Assessment of Natural Groundwater
Recharge in Upper Ganga Canal Command Area", Journal of Applied Hydrology,
Association of Hydrologists of India, Vol. XV, No. 4, October 2002, pp. 13-20.
9. Mishra, G. C., 1993. "Current Status of Methodology for Groundwater Assessment in
the Country in Different Region", National Institute of Hydrology, Technical Report No.
TR-140, 1992-93, 25 p.
10. National Water Policy, Ministry of Water Resources, Government of India, April
2002, 18 p.
11. Sokolov, A. A. and T. G. Chapman, 1974. "Methods for Water Balance Computations",
The UNESCO Press, Paris.
12. Sophocleous, Marios A., 1991. "Combining the Soilwater Balance and Water-Level
Fluctuation Methods to Estimate Natural Groundwater Recharge : Practical Aspects".
Journal of Hydrology, Vol. 124, pp. 229-241.
13. Thornthwaite, C. W., 1948. "An Approach towards a Rational Classification of
Climate". Geogr. Rev., Vol. 38, No. 1, pp. 55-94.
8/6/2019 Groundwater Assessment Met Bod Logy
21/21
21
14. Thornthwaite, C. W. and J. W. Mather, 1955. "The Water Balance". Publ. Climatol. Lab.
Climatol. Drexel Inst. Technol., Vol. 8, No. 1, pp. 1-104.